High-strength laminated sheet for optical applications

ABSTRACT

A laminated sheet includes a surface layer having an optical surface that is of fire-polished quality and a core layer having a higher modulus than the surface layer to increase an overall stiffness or fracture toughness of the laminated sheet.

BACKGROUND OF INVENTION

[0001] The invention relates generally to substrates, such as those usedin manufacturing flat panel displays, and method of making the same.

[0002] Display components, e.g., thin film transistors (TFTs), colorfilters, and organic light emitting diodes (OLEDs), are typicallyfabricated on glass substrates. Although, glass has desirable opticalproperties for display and other applications, it is relatively heavyand easily broken.

[0003] From the foregoing, there is desired a substrate for applicationssuch as displays, other active electronic devices, and optical devicepackaging that has a high mechanical strength and low thermal shrinkage.

SUMMARY OF INVENTION

[0004] In one aspect, the invention relates to a laminated sheet whichcomprises a surface layer having an optical surface that is offire-polished quality and a core layer having a higher modulus than thesurface layer to increase an overall stiffness of the laminated sheet.

[0005] In another aspect, the invention relates to a laminated sheetwhich comprises a compressive surface layer having an optical surfacethat is of fire-polished quality and a core layer, wherein the thermalexpansion coefficients of the surface and core layers are adjusted tocontrol stress and enhance fracture toughness of the laminated sheet.

[0006] In another aspect, the invention relates to a laminated sheetwhich comprises a surface layer having an optical surface that is offire-polished quality, a core layer having a higher modulus than thesurface layer to increase an overall stiffness of the laminated sheet,and a sacrificial layer interposed between the surface layer and thecore layer.

[0007] In another aspect, the invention relates to a laminated sheetwhich comprises a core layer, a surface layer disposed on a first sideof the core layer, the surface layer having an optical surface that isof fire-polished quality, and a bottom layer disposed on a second sideof the core layer, wherein the core layer has a higher section modulusthan the surface layer and the bottom layer to increase an overallstiffness of the laminated sheet.

[0008] In another aspect, the invention relates to a method of making alaminated sheet which comprises supplying a first viscous, flowablematerial into an overflow channel, overflowing the first viscous,flowable material in a controlled manner to form a first sheet-like flowhaving at least one untouched surface of fire-polished quality,depositing at least a second material on a surface of the firstsheet-like flow to form a laminated flow, and drawing the laminated flowinto a laminated sheet.

[0009] In another aspect, the invention relates to a method of making alaminated sheet which comprises supplying a viscous, flowable materialinto an overflow channel, positioning a consumable material in a flowpath of the viscous, flowable material so as to introduce acompositional variation to a specific portion of the viscous, flowablematerial, overflowing the viscous, flowable material in a controlledmanner to form a sheet-like flow having at least one untouched surfaceof fire-polished quality, and drawing the sheet-like flow into alaminated sheet.

[0010] In another aspect, the invention relates to a method of making alaminated sheet which comprises forming a first sheet-like flow by acontinuous sol-gel process, supplying a viscous, flowable material intoan overflow channel, overflowing the viscous, flowable material in acontrolled manner to form a second sheet-like flow, fusing the secondsheet-like flow with a surface of the first sheet-like flow to form alaminated flow, and drawing the laminated flow into a laminated sheet.

[0011] In another aspect, the invention relates to a method of making alaminated sheet which comprises supplying a first and a second viscous,flowable material into independent compartments in an overflow channel,simultaneously overflowing the first and second viscous, flowablematerials to form separate flow streams, uniting the separate flowstreams into a single sheet-like laminated flow, and drawing thesheet-like laminated flow into a laminated sheet.

[0012] In another aspect, the invention relates to a method of making alaminated sheet which comprises drawing a first viscous, flowablematerial through a slot to form a first sheet-like flow, supplying asecond viscous, flowable material into an overflow channel andoverflowing the second viscous, flowable material from the overflowchannel in a controlled manner to form a second sheet-like flow havingan untouched surface of fire-polished quality, merging the secondsheet-like flow with a surface of the first sheet-like flow to form alaminated flow, and drawing the laminated flow into a laminated sheet.

[0013] In another aspect, the invention relates to a method of making alaminated sheet which comprises supplying a viscous, flowable materialinto an open channel, simultaneously overflowing the viscous, flowablematerial from a first side of the open channel and drawing the viscous,flowable material from a second side of the open channel, wherein theviscous, flowable material drawn from the second side of the openchannel forms a sheet-like flow, wherein the viscous, flowable materialoverflowed from the first side of the open channel forms two separateflow streams having untouched surfaces of fire-polished quality, mergingeach of the flow streams with a surface of the sheet-like flow to form alaminated flow, and drawing the laminated flow into a laminated sheet.

[0014] In another aspect, the invention relates to a method of making alaminated sheet which comprises obtaining a sheet of material, supplyinga first viscous, flowable material to a first overflow channel,overflowing the first viscous, flowable material in a controlled mannerto form a sheet-like flow with an untouched surface of fire-polishedquality, and, while the sheet-like flow is in viscous, flowable form,merging the sheet-like flow with a surface of the sheet of material.

[0015] In another aspect, the invention relates to a method of making alaminated sheet which comprises supplying a first viscous, flowablematerial to a first overflow channel and a second viscous, flowablematerial to a second overflow channel, overflowing the first viscous,flowable material from the first overflow channel in a controlled mannerto form a first sheet-like flow with at least one untouched surface offire-polished quality, wherein the viscous, flowable material drawn fromthe second side of the open channel forms a sheet-like flow, wherein theviscous, flowable material overflowed from the first side of the openchannel forms two separate flow streams having untouched surfaces offire-polished quality, merging each of the flow streams with a surfaceof the sheet-like flow to form a laminated flow, drawing the laminatedflow into a laminated sheet.

[0016] In another aspect, the invention relates to an apparatus forforming a laminated sheet which comprises an overflow means for formingopposed sheet-like flows, wherein the overflow means has at least twoindependent compartments for receiving a viscous, flowable material, aforming means positioned below the overflow means for converging anduniting the opposed sheet-like flows into a single drawn sheet flow,means for pivotally adjusting the overflow means such that a surface ofthe overflow channel has a desired tilt angle with respect to thehorizontal, and means for delivering a viscous, flowable material intothe compartments.

[0017] In another aspect, the invention relates to an apparatus forforming a laminated sheet which comprises an overflow channel forforming opposed sheet-like flows from a viscous, flowable material, aslotted channel positioned below the overflow channel for forming adrawn sheet-like flow, and a forming body positioned below the overflowchannel for converging and uniting the opposed sheet-like flows withouter surfaces of the drawn sheet-like flow to form a laminated flow.The apparatus further includes means for pivotally adjusting theoverflow channel such that a surface of the overflow channel has adesired tilt angle with respect to the horizontal and means fordelivering a viscous, flowable material into the overflow channel.

[0018] Other features and advantages of the invention will be apparentfrom the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0019]FIG. 1 shows a two-layered laminated sheet according to oneembodiment of the invention.

[0020]FIG. 2 shows a coating layer added to the laminated sheet of FIG.1.

[0021]FIG. 3 shows a three-layered laminated sheet according to anotherembodiment of the invention.

[0022]FIG. 4 shows a coating layer added to the laminated sheet of FIG.3.

[0023]FIG. 5 shows a laminated sheet having a sandwich structureaccording to another embodiment of the invention.

[0024]FIG. 6A shows a perspective view of a double-sided overflow fusionpipe.

[0025]FIG. 6B shows a method for forming a sheet-like flow using thedouble-sided overflow fusion pipe of FIG. 6A.

[0026]FIG. 7A shows a perspective view of a single-sided overflow fusionpipe.

[0027]FIG. 7B shows a method for forming a sheet-like flow using thesingle-sided overflow fusion pipe of FIG. 7A.

[0028]FIG. 8A shows a perspective view of a multi-compartment overflowfusion pipe.

[0029]FIG. 8B shows a method for forming a laminated sheet-like flowusing the multi-compartment overflow fusion pipe of FIG. 8A.

[0030]FIG. 9A shows a device for drawing a sheet-like flow into a sheet.

[0031]FIG. 9B shows a sheet-like flow advancing into the device of FIG.9A.

[0032]FIG. 9C shows a side view of the device shown in FIG. 9A with asheet-like flow being conveyed through the channel of the device.

[0033]FIG. 9D shows a sheet emerging from the device of FIG. 9A.

[0034]FIG. 10 illustrates a method of forming a two-layered laminatedsheet using a double-sided overflow fusion pipe and a single-sidedoverflow fusion pipe.

[0035]FIG. 11 illustrates a method of forming a three-layered laminatedsheet using a double-sided overflow fusion pipe and two single-sidedoverflow fusion pipes.

[0036]FIG. 12 illustrates a method of forming a two-layered laminatedsheet using two single-sided overflow fusion pipes.

[0037]FIG. 13 illustrates a method of forming a two-layered laminatedsheet using two double-sided overflow fusion pipes.

[0038]FIG. 14 illustrates a method of forming a two-layered laminatedsheet using a single-sided overflow fusion pipe and a slot draw fusionpipe.

[0039]FIG. 15 illustrates a method of forming a two-layered laminatedsheet using a double-sided overflow fusion pipe with an integrated slotdraw fusion device.

[0040]FIG. 16A shows a perspective view of a double-sided overflowfusion pipe with a combined overflow and slot draw channel according toanother embodiment of the invention.

[0041]FIG. 16B is a vertical cross-section of the fusion pipe shown inFIG. 16A.

[0042]FIG. 17 illustrates a method of forming a two-layered laminatedsheet using a continuous sol-gel process and a single-sided overflowfusion pipe.

[0043]FIG. 18 illustrates a method of forming a sandwich laminated sheetstructure.

[0044]FIG. 19 illustrates a method of applying a coating material on afusion-formed surface.

[0045]FIG. 20 shows a coating material delivered to a fusion-formedsurface via an overflow channel.

[0046]FIG. 21 shows a coating material being sprayed on a fusion-formedsurface.

[0047]FIGS. 22A and 22B show a mapping of locations in a viscous,flowable material carried through a delivery system to locations in afinal sheet formed from the viscous, flowable material.

DETAILED DESCRIPTION

[0048] The invention will now be described in detail with reference to afew preferred embodiments, as illustrated in accompanying drawings. Inthe following description, numerous specific details are set forth inorder to provide a thorough understanding of the invention. It will beapparent, however, to one skilled in the art, that the invention may bepracticed without some or all of these specific details. In otherinstances, well-known features and/or process steps have not beendescribed in detail in order to not unnecessarily obscure the invention.The features and advantages of the invention may be better understoodwith reference to the drawings and discussions that follow.

[0049] The inventors propose herein a substrate that can be used forfabricating displays, especially flat panel displays. The substrate mayalso be used for other applications benefiting from a surface with highoptical flatness, such as active electronic devices, photovoltaicdevices, and substrates for biological arrays. In one embodiment, thesubstrate is a laminated sheet having a surface layer that exhibitsoptimal properties for enduring direct chemical and thermal interactionwith display manufacturing processes and a core layer that has a higherelastic modulus than the surface layer for enhancing the overallmechanical stiffness of the laminated sheet. In one embodiment, thesurface layer has a fusion-formed optical surface that is pristine andof fire-polished quality and a uniform thickness and flatness thatsatisfy display application requirements. In one embodiment, the corelayer is separable from the surface layer, leaving a final product,i.e., the surface layer, that is thin and meets display applicationrequirements.

[0050] In general, the materials used in forming the layers in thelaminated sheet are selected such that the optical, electrical,chemical, and mechanical properties of the laminated sheet are optimizedfor the target application. For example, the materials can be selectedsuch that the laminated sheet has a low density and is flexible, tough,and transparent. For the fusion-formed layers, the layer-formingmaterials should have viscoelastic characteristics. Examples ofmaterials exhibiting viscoelastic characteristics are glass and polymer.The laminated sheet may be made of multiple materials with differentelastic moduli, which could include, but are not limited to, glasses andpolymers and/or their precursors such as curable monomers and oligomers,or multiple polymers, or glasses, including low-melting glasses,glass-ceramics, nanocomposites, and any combination thereof. Thelaminated sheet may include additional layers besides the surface layerand the higher modulus core layer. For example, the laminated sheet mayinclude interlayers with thermal expansion coefficients tailored tomanage residual stress upon cooling. As another example, should theoutermost surface layer require additional optical functionality, thepristine, fire-polished quality need not be maintained. For example, oneof ordinary skill in the art will recognize that optically functionallayers, such as microlens arrays or polarizers, could be added to any ofthe laminated sheets described herein. One of ordinary skill in the artwill also recognize that the laminated sheet with fire-polished surfacequality could also be achieved with a redraw-type process, for example,redrawing of a slot-drawn laminated sheet.

[0051] Specific laminated sheet structures will now be described withreference to the accompanying drawings.

Two-Layered Laminated Sheet

[0052]FIG. 1 shows a two-layered laminated sheet 10 having a surfacelayer 12 and a core layer 14. The surface layer 12 and the core layer 14may be single layers, as illustrated in the figure, or may be made ofsub-layers. The surface layer 12 has a fusion-formed optical surface 16that can serve as a basis for fabricating functional elements (notshown), such as, but not limited to, TFTs, OLEDs, or color filters. Thefusion-formed optical surface 16 is untouched by a forming device, e.g.,uncontaminated by refractory material, and has a fire-polished surfacequality. The surface layer 12 has a uniform thickness and a flatnessthat meet display application requirements. The core layer 14 has ahigher elastic modulus than the surface layer 12 and provides mechanicalsupport to the surface layer 12 such that the structural integrity ofthe laminated sheet 10 is maintained while packaging and fabricatingfunctional elements (not shown) on the surface layer 12.

[0053] Preferably, the outer extent of the core layer 14 is placed asfar as possible from the neutral axis of the thickness of the laminatedsheet 10. This is because the section modulus of the laminated sheet 10depends on the square of the distance of the higher modulus layer 14from the neutral axis. Thus, the farther away the outer extent of thecore layer 14 is from the neutral axis of the thickness of the laminatedsheet, the higher the section modulus and the mechanical stiffness ofthe laminated sheet 10.

[0054] In one embodiment, the surface and core layers 12, 14 are made ofglasses and/or polymers or glasses and/or glass-ceramics. Examples ofglasses include, but are not limited to, alkaline-earthboro-aluminosilicate, zinc borosilicate, and soda-lime glass. Examplesof polymers include, but are not limited to, poly perfluorocyclobutaneether, polycyclic olefin, polyamide, polyether-sulfone (PES),polyarylate, polycarbonate, poly ethylene terphthalate (PET), polyethylene naphthalate (PEN), poly methyl methacrylate (PMMA), and polyvinyl butyral (PVB). Examples of glass-ceramics include, but are notlimited to, glasses enriched with magnesium oxide, yttria, berylia,alumina, or zirconia.

[0055] Preferably, the materials used in forming the surface and corelayers 12, 14 are transparent. The type of materials used in forming thesurface and core layers 12, 14 can be the same or can be different. Forexample, the surface and core layers 12, 14 may both be made of glass orof polymer. Alternatively, the surface layer 12 may be made of glass andthe core layer 14 of polymer, and vice-versa. Alternatively, the surfacelayer 12 may be made of glass and the core layer 14 made ofglass-ceramic. Typically, the layer-forming materials are selected basedon the display application requirements. For example, displayapplications involving fabrication of low-temperature poly-silicon(LTPS) TFTs typically require a substrate that has a high strain point,e.g., greater than 600° C., in order to minimize shrinking and/orwarping when the substrate is heated. These applications also generallyrequire a substrate having a thermal behavior that is similar to that ofsilicon.

[0056] In general, the materials used in forming the surface and corelayers 12, 14 will be selected to achieve a desired thermal expansionbehavior when the laminated sheet 10 is heated. Of particular interestin display applications is to select the materials such that the overallshrinkage of the laminated sheet 10 due to heating is negligible. Thelayer-forming materials may also be selected to achieve low density,flexibility, toughness, transparency, and, possibly, low permeation tomoisture and gas. The optical properties of the surface layer 12 can beoptimized for the target application, while the mechanical properties ofthe core layer 14 can be optimized to achieve, for example, a desiredtoughness and flexibility.

[0057] In one embodiment, the core layer 14 is separable from thesurface layer 12 by means such as thermal, chemical, mechanical,electromagnetic radiation, or other means. In one embodiment, separationof the core layer 14 from the surface layer 12 is achieved by selectingthe material used in forming the core layer 14 to be more soluble in anappropriate solvent than the material used in forming the surface layer12. The advantages of making the core layer 14 separable from thesurface layer 12 are significant. The surface layer 12 can be made asthin as necessary to meet display application requirements, e.g.,thinner than 1 mm. During handling and packaging of the laminated sheet10 and fabrication of functional elements on the surface layer 12, thecore layer 14 would provide the required mechanical support to thesurface layer 12. After adding function to the surface layer 12, thecore layer 14 can then be separated from the surface layer 12, leavingthe thin surface layer 12 that meets display application requirements.

[0058] When the core layer 14 is separable from the surface layer 12,there is a wider latitude in selecting the material used in the corelayer 14, i.e., the core layer 14 does not have to satisfy the stringentrequirements required for display applications since it will not be apart of the final product. Thus, for example, the core layer 14 may bemade of a cheap glass or polymer or glass-ceramic that provides thedesired mechanical stiffness to the surface layer 12. The core layer 14may also be made of a material that has a low thermal shrinkage, e.g., awell-annealed glass. Alternatively, the core layer 14 may be made of amaterial that can be expanded to achieve zero thermal shrinkage, e.g., afoam material such as polystyrene. The core layer 14 can be made asthick as necessary to provide the necessary mechanical support to thesurface layer 12 and does not need to have a fusion-formed opticalsurface that is pristine and of fire-polished quality.

[0059] One or more layers of coatings can be added to the laminatedsheet 10. For example, FIG. 2 shows a coating layer 18 formed on theoptical surface 16 of the laminated sheet 10. The coating layer 18 couldbe made of a metal, semiconducting, or nonmetallic organic or inorganicmaterial. In one embodiment, the coating layer 18 could act as a barrierto the environment, e.g., act to reduce gas and moisture permeation. Aprotective coating (not shown) may also be added to the core layer 14.Other types of coating may also be added to the laminated sheet 10, suchas an optical coating, which will enhance the optical properties of thesurface layer 12 and/or core layer 14, or a conductive coating, whichcan be used as a basis for fabricating active electronic devices, suchas electrodes, diodes, or transistors, or a dark or dark color coatingthat can enhance the contrast of the display, or a reflective metalcoating that can enhance the brightness of the display, etc.

Three-Layered Laminated Sheet

[0060]FIG. 3 shows a three-layered laminated sheet 20 which includes asurface layer 22, a sacrificial interlayer 24, and a core layer 26. Thesurface layer 22 and the core layer 26 may be single layers, asillustrated in the figure, or may be made of sub-layers. The surfacelayer 22 has a fusion-formed optical surface 28 that can serve as abasis for fabricating active and passive electronic devices. Thefusion-formed optical surface 28 is untouched by a forming device, e.g.,uncontaminated by refractory material, and has a fire-polished surfacequality. The core layer 26 has a higher elastic modulus than the surfacelayer 22 and provides mechanical support to the surface layer 22, aspreviously described above. Much of the discussions above concerningmaterials to use in the two-layered laminated sheet (10 in FIGS. 1 and2) also apply to the three-layered laminated sheet 20.

[0061] The sacrificial interlayer 24 couples the surface layer 22 to thecore layer 26 and may be removed so as to allow the surface layer 22 tobe separated from the core layer 26. For example, the sacrificialinterlayer 24 may be made of a material that is more soluble in anappropriate solvent than at least the material used in the surface layer22. In this way, the core layer 26 can be separated from the surfacelayer 22 by dissolving the sacrificial interlayer 24 away. The advantageof this embodiment over the one shown in FIG. I is that the sacrificialinterlayer 24 can be made very thin so that the amount of material to bedissolved away is very small. In this way, the core layer 26 can be madeas thick as necessary to provide the necessary mechanical support to thesurface layer 22 while it is coupled to the surface layer 22 through thesacrificial interlayer 24.

[0062] One or more layers of coatings may be added to the laminatedsheet 20. For example, FIG. 4 shows a coating layer 30 formed on theoptical surface 28 of the laminated sheet 20. The coating layer 30 couldbe made of a metal, semiconducting, or nonmetallic organic or inorganicmaterial. In one embodiment, the coating layer 30 could act as a barrierto the environment, e.g., act to reduce gas and moisture permeation. Aprotective coating (not shown) may also be added to the core layer 26.Other types of coating may also be added to the laminated sheet 20, suchas an optical coating, which will enhance the optical properties of thesurface layer 22 and/or core layer 26, or a conductive coating, whichcan be used as a basis for fabricating active electronic devices, suchas electrodes, diodes, or transistors, or a dark or dark color coatingthat can enhance the contrast of the display, or a reflective metalcoating that can enhance the brightness of the display, etc.

Sandwich Laminated Sheet

[0063]FIG. 5 shows a laminated sheet 32 having a surface layer 34, acore layer 36, and a bottom layer 38, where the core layer 36 issandwiched between the surface layer 34 and the bottom layer 38. Thesurface layer 34, core layer 36, and bottom layer 38 may be singlelayers, as illustrated in the figure, or may be made of sub-layers. Thesurface layer 34 has a fusion-formed optical surface 40 that can serveas a basis for fabricating functional elements (not shown), such asTFTs, OLEDs, or color filters. The fusion-formed optical surface 40 isuntouched by a forming device, e.g., uncontaminated by refractorymaterial, and has a fire-polished surface quality. The bottom layer 38is generally non-functional and does not necessarily have to meet thestringent requirements for the surface layer 34. The surface and bottomlayers 34, 38 can be made very thin. The core layer 36 can be made muchthicker than the surface and bottom layers 34, 38 and may have a highersection modulus than the surface and bottom layers 34, 38 so as toimprove the overall structural integrity of the laminated sheet 32. Thecore layer 36 can be made of a low thermal shrinkage material that willconstrain the shrinkage of the laminated sheet 34 while undergoingcyclic thermal history.

[0064] Generally speaking, the higher strength of the laminated sheet 32comes from the surface layer 34 that is resistant to compression, thebottom layer 38 that is resistant to tension, and the core layer 36 thatis resistant to shear stress. In one embodiment, the thermal expansioncoefficients of the surface layer 34, the core layer 36, and the bottomlayer 38 are adjusted to control stress and enhance fracture toughnessof the laminated sheet 32. If desired, function can be added to the corelayer 36. For example, the core layer 36 can be optically enhanced toenhance the brightness and/or enlarge viewing angle of the display. Asdescribed for the two-layered and three-layered laminated sheets above,one or more coating layers (not shown), e.g., protective coating,optical coating, or conductive coating, may be added to the surfacelayer 34 and/or the bottom layer 38.

Methods for Making Laminated Sheets

[0065] The inventors propose herein a method of making a laminated sheethaving at least one fusion-formed optical surface that is pristine andof fire-polished quality. The method of the invention can also be usedto add one or more layers of coating to the laminated sheet. The coatinglayer may be protective, e.g., reduce gas and moisture permeation. Thecoating layer may enhance the optical properties of the substrate, e.g.,enhance brightness or enlarge viewing angle. The coating layer may beconductive or have other functional characteristics

[0066] In general, the methods for making the laminated sheets of theinvention start with formation of a sheet of material by a fusionprocess. In the fusion process, a viscous, flowable material flows intoan overflow channel and then overflows in a controlled manner from oneside or both sides of the overflow channel, depending on theconfiguration of the fusion-forming device, to form a sheet-like flow.The other layers of the laminated sheet may be formed by depositingother materials on one or both surfaces of the sheet-like flow. Thethickness of the layers are controlled to achieve a laminated sheethaving a desired thickness.

[0067] The viscous, flowable material(s) used in forming the layers ofthe laminated sheets may be materials such as, but not limited to, aglass melt or polymer melt. In some embodiments, the viscous, flowablematerial(s) may be a base glass melt that can, with the addition of oneor more elements or oxides, be readily transformed into several specificglasses. The elements or oxides could be nucleating agents for aglass-ceramic, with the base glass being the precursor glass withoutthese agents or with these agents below a critical composition level.The elements or oxides could also be glass components that would cause asignificant, controlled change in the resulting elastic modulus of theformed layer.

[0068] Recent advances in modeling of the fusion process usingcomputational fluid dynamics make possible the mapping of the preciselocation in the delivery system of the glass (or polymer) thatultimately ends up at any location in the finished sheet. Thus, elementsor oxides can be introduced to the glass melt according to acomputer-model-generated map to produce a final fusion drawn sheethaving the desired layered structure. The oxides or elements could beintroduced, for example, by positioning blocks of consumable(dissolvable) material containing the oxides or elements at specificpositions in the fusion pipe to achieve the desired layered structure.

Double-Sided Overflow Fusion Pipe

[0069]FIG. 6A shows a double-sided overflow fusion pipe 42 having awedge-shaped forming body 44 bounded by converging sidewalls 46, 48. Anoverflow channel 50 is formed in the upper portion of the forming body44. The overflow channel 50 is bounded by sidewalls (or dams) 52, 54 anda contoured bottom surface 56. The shape of the contoured bottom surface56 is such that the height of the overflow channel 50 decreases as itextends outwardly from the channel inlet 58 to the dam 60. In operation,a delivery pipe (not shown) would be connected to the channel inlet 58for delivery of a viscous, flowable material to the overflow channel 50.

[0070] The fusion pipe 42 can be pivotally adjusted by any suitablemeans, such as roller, wedge, or cam 62, such that the upper surfaces ofthe dams 52, 54 have a desired tilt angle with respect to thehorizontal. The tilt angle of the dams 52, 54, the rate at which aviscous, flowable material is supplied to the overflow channel 50, andthe viscosity of the flowable material can be selected such that asingle sheet-like flow having a uniform thickness is formed.

[0071] In operation, a viscoelastic material, e.g., a glass or polymermaterial, is blended, melted, and stirred well. As shown in FIG. 6B, thehomogeneous flowable viscoelastic material 64 is then delivered to thechannel inlet 58. The flowable viscoelastic material 64 has a loweffective head that allows it to flow up the overflow channel 50 withoutsurge or agitation. The flowable viscoelastic material 64 wells over thedams 52, 54 and flows down the converging sidewalls 46, 48 of theforming body 44 as flow streams 64 a, 64 b. At the bottom of the formingbody 44, the separate flow streams 64 a, 64 b rejoin to form a singlesheet-like flow 66 with pristine surfaces 66 a, 66 b of fire-polishedsurface quality.

[0072] The initial thickness of the sheet-like flow 66 is determined bythe amount of material flowing down the converging walls 46, 48, wherethe amount of material 64 a, 64 b flowing down the converging walls 46,48 is dependent on the head pressure of the flowable viscoelasticmaterial 64 and the geometry of the overflow channel 50.

Single-Sided Overflow Fusion Pipe

[0073]FIG. 7A shows a single-sided overflow fusion pipe 70 having awedge-shaped forming body 72 bounded by a straight sidewall 74 and aconverging sidewall 76. An overflow channel 78 is formed in the upperportion of the forming body 72. The overflow channel 78 is bounded bysidewalls (or dams) 82, 84 and a contoured bottom surface 86. The dam 82is higher than the dam 84, so that overflow occurs only at the dam 84.In operation, a delivery pipe (not shown) would be connected to theinlet 88 of the overflow channel 78 for delivery of a viscous, flowablematerial to the overflow channel 78. The shape of the contoured bottomsurface 86 is such that the height of the overflow channel 78 decreasesas it extends outwardly from the inlet 88 to the dam 80.

[0074] The fusion pipe 70 can be pivotally adjusted by any suitablemeans, such as roller, wedge, or cam 90, such that the upper surfaces ofthe dams 82, 84 have a desired tilt angle with respect to thehorizontal. The tilt angle of the dams 82, 84, the rate at which aviscous, flowable material is supplied to the overflow channel 78, andthe viscosity of the flowable material can be selected such that asingle sheet-like flow having a uniform thickness is formed.

[0075] In operation, a viscoelastic material, e.g., a glass or polymermaterial, is blended, melted, and stirred well. As shown in FIG. 7B, thehomogeneous flowable viscoelastic material 102 is then delivered to thechannel inlet 88. The flowable viscoelastic material 102 has a loweffective head that allows it to flow into the overflow channel 78without surge or agitation. The flowable viscoelastic material 102 wellsover the dam 84 and flows down the converging sidewall 76 of the formingbody 72 to form a single sheet-like flow 104 with pristine surface 104 aof fire-polished surface quality. The surface 104 b is not pristinebecause it makes contact with the converging sidewall 76.

[0076] The initial thickness of the sheet-like flow 104 is determined bythe amount of material 102 flowing down the converging sidewall 76,where the amount of material 102 flowing down the converging sidewall 76is dependent on the head pressure of the flowable viscoelastic material102 and the geometry of the overflow channel 78.

Multi-Compartment Overflow Fusion Pipe

[0077]FIG. 8A shows an overflow fusion pipe 106 having a wedge-shapedforming body 107 bounded by converging sidewalls 108, 110. Independentoverflow channels 112, 114 are formed in the upper portion of theforming body 107. The overflow channel 112 is bounded by sidewalls (ordams) 118, 120 and a contoured bottom surface 122. The dam 120 is higherthan the dam 118 so that overflow occurs only at the dam 118. Theoverflow channel 114 is bounded by sidewalls (or dams) 120, 124 and acontoured bottom surface 126. The dam 120 is higher than the dam 124 sothat overflow occurs only at the dam 124. In operation, delivery pipeswould be connected to the inlets 128, 129 of the overflow channels 112,114, respectively. The shapes of the contoured bottom surfaces 122, 126are such that the height of the overflow channels 112, 114 decrease asthey extend outwardly from the inlets 128, 129 to the dam 131.

[0078] In operation, as shown in FIG. 8B, a first flowable viscoelasticmaterial 130 is delivered to the channel inlet (128 in FIG. 8A) and asecond flowable viscoelastic material 132 is delivered to the channelinlet (129 in FIG. 8B). The flowable viscoelastic materials 130, 132have a low effective head that allows them to flow into the overflowchannels 112, 114, respectively, without surge or agitation. Theflowable viscoelastic materials 130, 132 well over the dams 118, 124,respectively, and flow down the converging sidewalls 108, 110 of theforming body 107. At the bottom of the forming body 107, the materials130, 132 merge to form a laminated, sheet-like flow 134 with pristinesurfaces 134 a, 134 b of fire-polished surface quality.

[0079] The initial thickness of the sheet-like flow 134 is determined bythe amount of materials 130, 132 flowing down the converging sidewalls108, 110, where the amount of materials 130, 132 flowing down theconverging sidewalls 108, 110 are dependent on the head pressure of theviscous, flowable materials 130, 132 and the geometry of the overflowchannels 112, 114.

Down Draw Process

[0080] Various types of overflow fusion pipes have been described abovefor forming a sheet-like flow. The fusion process further includesdrawing the sheet-like flow into a sheet with a desired thickness. FIG.9A shows a drawing device 140 according to an embodiment of theinvention. The drawing device 140 includes a channel 142 for receiving asheet-like flow (not shown). The channel 142 is shown as vertical butcould also have some other orientation, e.g., horizontal. A verticalchannel is generally preferred because drawing is favored under verticalconditions due to the effect of gravity. The width of the channel 142determines the width of the final sheet (not shown). Typically, thecross-draw dimension of the final sheet is greater than about 4 feet.However, a sheet having a small cross-draw dimension may also be formed.

[0081] Sets of rollers (or edge guides) 144 are positioned along thelength of the channel 142 to convey the sheet-like flow (not shown)through the channel 142 and to control (attenuate) the thickness of thesheet-like flow. A series of heated zones 146 are defined inside thechannel 142. The heated zones 146 become progressively cooler in thedirection shown by the arrow 148. The zones 146 may be heated byelectrical heating elements, induction heaters, or other heating means(not shown).

[0082]FIG. 9B shows a sheet-like flow 150 advancing from a fusion pipe152 into the channel 142. The sheet-like flow 150 may be guided into thechannel 142 by means of edge guides (not shown). Referring to FIG. 9C,inside the channel 142, paired rollers 144 gently grip (or pressagainst) the vertical edges of the sheet-like flow 150 and convey thesheet-like flow 150 down the channel 142. The spacing between each ofthe paired rollers 144 is such that the thickness of the sheet-like flow150 is gradually reduced as the sheet-like flow 150 is conveyed down thechannel 142. If necessary, temperature can be used to “tweak” thethickness of the sheet-like flow 150, i.e., a hot or cold spot can beapplied to selected regions of the flow to thin or thicken the flow 150.

[0083] As the sheet-like flow 150 is conveyed through the cooler regionsof the heated zones 146, it is formed into a very flat and uniformsheet. FIG. 9D shows the sheet 154 coming out of the channel 142. Thesheet 154 can be scored and cut as necessary, Typically, the verticaledges of the sheet 154, which have been in contact with the rollers 144,would have to be trimmed off because they are not pristine.

Method of Forming a Two-Layered Laminated Sheet using a Single-Sided anda Double-Sided Overflow Fusion Pipe

[0084]FIG. 10 shows an apparatus 156 for making a two-layered laminatedsheet. The apparatus 156 includes the double-sided overflow fusion pipe42 (previously shown in FIG. 6A) and the single-sided overflow fusionpipe 70 (previously shown in FIG. 7A). The double-sided overflow fusionpipe 42 is placed above the single-sided overflow fusion pipe 70. Ingeneral, the fusion pipes 42, 70 are arranged in a manner that allowsthe sheet-like flow produced by the fusion pipe 42 to be merged with thesheet-like flow produced by the fusion pipe 70.

[0085] In operation, the overflow channel 78 in the fusion pipe 42 isfilled with a first flowable viscoelastic material 158, e.g., a glass orpolymer material. The flowable viscoelastic material 158 wells over thedams 52, 54 and flows down the converging sidewalls 46, 48 as separateflow streams 158 a, 158 b. The flow streams 158 a , 158 b rejoin at thebottom of the fusion pipe 42 to form a single sheet-like flow 160.

[0086] Similarly, the overflow channel 78 in the fusion pipe 70 isfilled with a second flowable viscoelastic material 162, e.g., a glassor polymer material. The viscous, flowable material 162 wells over thedam 84 and flows down the converging sidewall 76 to form a singlesheet-like flow 162. The single sheet-like flow 162 merges with thesurface 160 a of the single sheet-like flow 160 to form a laminated flow164. The surfaces 164 a, 164 b of the laminated flow 164 are pristineand of fire-polished surface quality. The laminated flow 164 is thendrawn into a sheet using the drawing device (140 in FIG. 9A).

[0087] It should be noted that either of the surfaces 164 a, 164 b canserve as a basis for forming functional elements because they are bothpristine. In general, the final thickness and optical characteristics ofeach layer of the laminated sheet will determine whether the layer canact as a surface layer or a core layer.

Method of Forming a Three-Layered Laminated Sheet using Two Single-Sidedand a Double-Sided Overflow Fusion Pipes

[0088] The apparatus 156 shown in FIG. 10 can be easily modified to forma three-layered sheet. As shown in FIG. 11, another single-sidedoverflow fusion pipe 70 b can be used to form a single sheet-like flow170, which can then be merged with the surface 164 a of the sheet-likeflow 164 to form a three-layered laminated flow 168. The laminated flow168 can be drawn into a sheet using the drawing device (140 in FIG. 9A).One or more layers can be added to the laminated flow 168 usingadditional single-sided overflow fusion pipes, positioned below theoverflow fusion pipes 70, 70 b.

Method of Forming a Two-Layered Laminated Sheet using Two Single-SidedOverflow Fusion Pipes

[0089]FIG. 12 shows an apparatus 166 for making a two-layered laminatedsheet. The apparatus 166 includes two of the single-sided overflowfusion pipes 70 (previously shown in FIG. 5), herein identified asfusion pipes 70R and 70L. The fusion pipes 70R and 70L are positioned atthe same height. However, if desired, one fusion pipe can be elevatedabove the other. In general, the fusion pipes 70R and 70L are positionedsuch that the sheet-like flows they produce merge to form a singlelaminated flow.

[0090] In operation, a first viscoelastic material 174, e.g., a glass orpolymer material, is supplied to the overflow channel 78L. At the sametime, a second viscoelastic material 176, e.g., a glass or polymermaterial, is supplied to the overflow channel 78R. The viscoelasticmaterials 174, 176 well over the dams 84L, 84R, respectively, and flowdown the converging sidewalls 76L, 76R, respectively, to form the singlesheet-like flows 178, 180, respectively. The single-sheet like flows178, 180 merge to form a laminated flow 182 with pristine surfaces 182a, 182 b of fire-polished surface quality. The laminated flow 182 isthen drawn into a sheet using the drawing device (140 in FIG. 9A).Layers can be added to the laminated flow 182 by positioning one or moresingle-sided overflow fusion pipes below the fusion pipes 70L, 70R.

[0091] Again, either of the surfaces 182 a, 182 b can serve as a basisfor forming functional elements because they are both pristine. Ingeneral, the final thickness and optical characteristics of each layerof the laminated sheet will determine whether the layer can act as asurface layer or a core layer

Method of Forming a Two-Layered Laminated Sheet Using Two Double-SidedOverflow Fusion Pipes

[0092]FIG. 13 shows an apparatus 184 for making a two-layered laminatedsheet. The apparatus 184 includes two of the double-sided overflowfusion pipes 42 (previously shown in FIG. 6A), herein identified asfusion pipes 42R and 42L. The fusion pipes 42R 42L are spaced aparthorizontally. The fusion pipe 42L is elevated above the fusion pipe 42R.Below the fusion pipes 42R, 42L is a float bath 186. The fusion pipes42R, 42L are positioned such that the sheet-like flows they produce canbe transitioned from the vertical to the horizontal and floated on thefloat bath 186. Edge guiding devices 192, 194, such as rollers, could beprovided to guide the sheet-like flows from the vertical orientation tothe horizontal orientation. The float bath 186 could be a bed of air,molten tin, molten polymer, or other floating material that iscompatible with the materials that will be used in forming the laminatedsheet.

[0093] In operation, the fusion pipes 42L, 42R are filled with viscous,flowable materials 187, 189, respectively. The viscous, flowablematerials 187, 189 overflow the dams 52L, 54L and 52R, 54R,respectively, to form the sheet-like flows 188, 190. The sheet-like flow188 is guided to the float 186 by edge guides 192 and floated on thefloat bath 186. The sheet-like flow 190 is guided to the sheet-like flow188 by edge guide(s) 194 and merged with the sheet-like flow 188 to forma laminated flow 195. The laminated flow 195 can be transitioned fromthe float bath 186 into the drawing device (140 in FIG. 9A) for drawinginto a laminated sheet. Additional double-sided overflow fusion pipesand/or single-sided overflow fusion pipes can be added to the apparatus184 to make a laminated sheet with more than two layers.

Method of Forming a Two-Layered Laminated Sheet using a Single-OverflowFusion Pipe and a Slot Draw Fusion Pipe

[0094]FIG. 14 shows an apparatus 196 for making a two-layered laminatedsheet. The apparatus 196 includes a single-sided overflow fusion pipe 70and a slot draw device 198. The slot draw device 198 includes a formingbody 200 having a channel 202 for receiving a viscous, flowable material204. The forming body 200 also includes a longitudinal slot 206 which isconnected to the channel 202. The viscous, flowable material 204 in thechannel 202 flows down the slot 206 to form a sheet-like flow 208. Theoverflow channel 78 in the fusion pipe 70 is also filled with a viscous,flowable material 210, which wells over the dam 84 and flows down theconverging sidewall 76 to form a sheet-like flow 212. The sheet-likeflows 208, 212 merge to form a laminated flow 214 with a pristinesurface 214 b. The other surface 214 a is not pristine because itcontacts the slot draw device 198 as the sheet-like flow 208 flows downthe slot 206. The laminated flow 214 can be drawn into a laminated sheetas previously described.

Method for Forming a Two-Layered Laminated Sheet using a Double-SidedOverflow Fusion Pipe with a Slot Draw Device

[0095]FIG. 15 shows another type of double-sided overflow fusion pipe216 that allows a laminated sheet to be formed via a combination of afusion process and a slot draw process. The fusion pipe 216 has aforming body 218 bounded by converging sidewalls 220, 222. An overflowchannel 224 is formed in the upper portion of the forming body 218. Achannel 226 is formed in the base of the forming body 218. The channel226 opens to the bottom of the forming body 218 through a longitudinalslot 228. Viscous, flowable material 230 in the overflow channel 224wells over the dams 232, 234 and flows down the converging walls 220,222 of the forming body 218 as flow streams 230 a, 230 b, respectively.Viscous, flowable material 236 in the channel 226 flows down thelongitudinal slot 228 as a sheet-like flow 238. The separate streams 230a, 230 b merge with the surfaces 238 a, 238 b of the sheet-like flow 238to form a laminated flow 240.

[0096]FIG. 16B shows another type of double-sided overflow fusion pipe215 that allows a laminated sheet to be formed via a combination of afusion process and a slot draw process. The fusion pipe 215 has aforming body 217. A channel 219 is formed in the forming body 217. Asshown in FIG. 16B, the channel 219 runs through the forming body 217 andhas an opening 221 at the top of the forming body 217 and an opening 223at the bottom of the forming body 217. Returning to FIG. 16A, thechannel 219 is bounded by sidewalls (or dams) 225, 227, 229. An inlet231 is provided for feeding a viscous, flowable material 233 into thechannel 219.

[0097] In operation, the viscous, flowable material 233 issimultaneously flowed over the dams 225, 227 and drained (or drawn)through the opening 223. The viscous, flowable material drained throughthe opening 223 forms a sheet-like flow 235. The viscous, flowablematerial flowed over the dams 225, 227 forms two separate flow streams237, 239, which are merged with the surfaces of the sheet-like flow 235to form a three-layered laminated flow 241. The laminated flow 241 canbe drawn into a sheet using the drawing device (140 in FIG. 9A). Notethat in this case, all the layers of the laminated flow 241 are made ofthe same material, but the center core layer (formed by the sheet-likeflow 235) can be much thicker than the outer layers (formed by the flowstreams 237, 239) to provide the laminated sheet with enhancedmechanical stiffness.

Method of Forming a Two-Layered Laminated Sheet Using a ContinuousSol-Gel Process and a Single-Sided Overflow Fusion Pipe

[0098] In FIG. 17, a continuous sol-gel process, indicated as 242, isused to produce an aerogel 244, which forms the core of a laminatedsheet. The aerogel 244 has a continuous porosity and can be prepared asa transparent, porous solid. A single-sided overflow fusion pipe 70 isthen used to form a sheet-like flow 246 which is merged with a surface244 a of the aerogel 244 to form a two-layered laminated flow 248, whichcan be drawn into a sheet using the drawing device (140 in FIG. 9A). Toform a sandwich (or three-layered) laminated sheet, another single-sidedoverflow fusion pipe (not shown) can be used to form a sheet-like flow(not shown), which can be merged with the surface 244 b of the aerogel244.

[0099] The sol-gel process is well-known. The process generally involvespreparing a “sol” using inorganic metal salts or metal organic compoundssuch as metal alkoxides. The starting materials are typically subjectedto a series of hydrolysis and condensation reactions to form a colloidalsuspension, i.e., the “sol.” The sol undergoes a transition to a softporous mass, called a “wet gel.” The liquid in the wet gel can beremoved by either air drying or supercritical extraction to form theaerogel. The aerogel is typically rigid. The aerogel can be madeflexible by adding fibers to the sol, allowing the fiber-reinforced solto gel, and extracting liquid from the gel.

Method of Making a Sandwich Laminated Sheet

[0100] The methods described above can be used to form the sandwichlaminated structure illustrated in FIG. 5. In another embodiment, thecore of the sandwich laminated structure is prepared separately. Then,fusion processes are used to add layers to the core. One motivation forforming the core separately is to allow for additional steps to enhancethe properties, such as mechanical and thermal properties, of the core.For example, the core of the sandwich structure may be a glass materialthat is subjected to annealing steps prior to forming of the sandwichstructure.

[0101]FIG. 18 shows a core substrate 250 interposed between twosingle-sided overflow fusion pipes 70R, 70L. Viscous, flowable materials252, 254 are supplied to the fusion pipes 70R, 70L and allowed tooverflow in a controlled manner to form sheet-like flows 256, 258,respectively. The sheet-like flows 256, 258 merge with the surfaces 250a, 250 b of the core substrate 250 to form the sandwich structure, whichis drawn to the desired thickness.

Method for Applying Coating to a Fusion-Formed Surface

[0102] In FIG. 19, the double-sided overflow fusion pipe 42 (previouslyshown in FIG. 6A) is used to form a sheet-like flow 262 with pristinesurfaces 262 a, 262 b. While the sheet-like flow 262 is still inviscous, flowable form, a slot/trough type delivery system 264 is usedto deliver a coating material 266 to the surface 262 a of the sheet-likeflow 262.

[0103] Other delivery systems besides the slot/trough type deliverysystem 264 can also be used to deliver the coating material 266. Forexample, FIG. 20 shows the coating material 266 being delivered via anoverflow channel 268. The coating material 266 is delivered into thechannel 268 and allowed to overflow in a controlled manner over the dam270. The dam 270 is positioned relative to the surface 262 a such thatthe coating material 266 merges with the surface 262 a as it overflowsfrom the dam 270. Another overflow channel 268 b (or other deliverysystem) can be provided to coat the other surface 262 b of thesheet-like flow 262.

[0104] Another method for coating the fusion-formed surface 262 ainvolves spraying the coating material on the fusion-formed surface 262a. FIG. 21 shows the coating material 266 being sprayed on the surface262 a of the sheet-like flow 262. The coating material 266 can besprayed while drawing down the sheet-like flow 262. If the coatingmaterial 266 is a polymer, it can be subsequently cured, as indicated at272, via radiation, microwave, or heat.

[0105] For all the coating methods described above, the sheet-like flow262 is very reactive when in flowable or molten form, allowing thecoating material 266 to form a strong bond with the surface(s) of thesheet-like flow 266 without the use of an adhesive.

Method of Making a Laminated Sheet with a Glass-Ceramic Layer

[0106] The methods described above can be used to form a laminated sheethaving a glass-ceramic layer with the modification that the nucleatingagents for the glass-ceramic layer are introduced to the delivery systemat appropriate locations. The nucleating agents can be introduced to theglass delivery system by positioning blocks of consumable material(containing the nucleating agents) at the appropriate locations in thedelivery system. In one embodiment, the glass-ceramic layer is a buriedwithin the laminated sheet. A method of forming such a buried laminatewill now be described.

[0107] As previously discussed, recent advances in modeling of thefusion process make it possible to map the precise location in thedelivery system that ultimately ends up at any location in the finishedsheet. FIG. 22A shows an example of such a map. The map 300 indicateswhere to position blocks of consumable material 302 in a melt 304 toform a desired layered structure. The line 306 demarcates the portion ofthe melt 304 that will ultimately end up on the surface of the formedsheet. The line 310 demarcates the portion of the melt 304 that will beburied in the formed sheet. FIG. 22B shows a surface glass composition312 corresponding to the surface layer of the formed sheet and a buriedglass composition 314 corresponding to the core layer of the formedsheet.

[0108] In operation, a melt having the compositional variation shown in,for example, the map 300 of FIGS. 22A and 22B would be delivered to adouble-sided overflow fusion pipe to form a sheet-like flow, aspreviously described. The final glass sheet would have a buried layerthat includes elements from the consumable material. The final glasssheet can then be subjected to a ceraming process to form the buriedglass-ceramic layer.

[0109] It should be noted that the method described above can generallybe used to form a laminated sheet having a buried layer with acomposition that is different from the surface layer, where theconsumable material is used to introduce the desired compositionalvariations.

[0110] An alternative process for forming a laminated sheet with aglass-ceramic layer involves forming the glass-ceramic layer separatelyand then using a fusion process to add a surface layer to theglass-ceramic layer in a manner similar to the one described above forthe sandwich laminated sheet.

[0111] Those skilled in the art will understand that variouscombinations of the methods described above can be used to formlaminated sheets.

[0112] While the invention has been described with respect to a limitednumber of embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A laminated sheet, comprising: a surface layerhaving an optical surface of fire-polished quality; and a core layerhaving a higher modulus than the surface layer to increase an overallstiffness of the laminated sheet.
 2. The laminated sheet of claim 1,wherein the optical surface is a fusion-formed surface.
 3. The laminatedsheet of claim 1 having a cross-draw dimension of greater than about 4feet.
 4. The laminated sheet of claim 1, wherein the core layer isseparable from the surface layer.
 5. The laminated sheet of claim 1,wherein a thickness of the surface layer is less than about 1 mm.
 6. Thelaminated sheet of claim 1, wherein the surface layer and the core layerare each made of a viscoelastic material.
 7. The laminated sheet ofclaim 6, wherein the viscoelastic material is selected from the groupconsisting of glasses and polymers.
 8. The laminated sheet of claim 1,wherein the surface layer comprises a glass.
 9. The laminated sheet ofclaim 8, wherein the core layer comprises a glass-ceramic.
 10. Thelaminated sheet of claim 9, wherein the core layer is buried within thesurface layer.
 11. The laminated sheet of claim 1, wherein the corelayer is made of a material having a low thermal shrinkage.
 12. Thelaminated sheet of claim 1, wherein the optical surface is coated with acoating material.
 13. The laminated sheet of claim 12, wherein thecoating material acts as a barrier to the environment.
 14. The laminatedsheet of claim 12, wherein the coating material enhances selectedoptical properties of the surface layer.
 15. The laminated sheet ofclaim 12, wherein the coating material comprises a conductive material.16. The laminated sheet of claim 12, wherein the coating materialcomprises a semiconductive material.
 17. The laminated sheet of claim 1,wherein the core layer is optically enhanced.
 18. A laminated sheet,comprising: a compressive surface layer having an optical surface thatis of fire-polished quality; and a core layer; wherein the thermalexpansion coefficients of the surface and core layers are adjusted tocontrol stress and enhance fracture toughness of the laminated sheet.19. The laminated sheet of claim 18, wherein the optical surface is afusion-formed surface.
 20. The laminated sheet of claim 18 having across-draw dimension of greater than about 4 feet.
 21. The laminatedsheet of claim 18, wherein a thickness of the surface layer is less thanabout 1 mm.
 22. The laminated sheet of claim 18, wherein the surfacelayer and the core layer are each made of a viscoelastic material. 23.The laminated sheet of claim 22, wherein the viscoelastic material isselected from the group consisting of glasses and polymers.
 24. Thelaminated sheet of claim 18, wherein the surface layer comprises aglass.
 25. The laminated sheet of claim 24, wherein the core layercomprises a glass-ceramic.
 26. A laminated sheet, comprising: a surfacelayer having an optical surface that is of fire-polished quality; a corelayer having a higher modulus than the surface layer to increase anoverall stiffness of the laminated sheet; and a sacrificial layerinterposed between the surface layer and the core layer.
 27. Thelaminated sheet of claim 26, wherein the optical surface is afusion-formed surface.
 28. The laminated sheet of claim 26, wherein thesurface, core, and sacrificial layers are each made of a viscoelasticmaterial.
 29. The laminated sheet of claim 28, wherein the viscoelasticmaterial is selected from a group consisting of glasses and polymers.30. The laminated sheet of claim 26, wherein the surface layer comprisesa glass, and the core layer comprises a glass-ceramic.
 31. The laminatedsheet of claim 26, wherein a thickness of the surface layer is less thanabout 1 mm.
 32. The laminated sheet of claim 26, wherein the opticalsurface is coated with a coating material.
 33. The laminated sheet ofclaim 32, wherein the coating material acts as a barrier to theenvironment.
 34. The laminated sheet of claim 32, wherein the coatingmaterial enhances selected optical properties of the surface layer. 35.The laminated sheet of claim 32, wherein the coating material comprisesa conductive material.
 36. The laminated sheet of claim 32, wherein thecoating material comprises a semiconductive material.
 37. The laminatedsheet of claim 26, wherein the core layer is optically enhanced.
 38. Thelaminated sheet of claim 26, wherein the sacrificial layer is moresoluble in a selected solvent than at least the surface layer.
 39. Alaminated sheet, comprising: a core layer; a surface layer disposed on afirst side of the core layer, the surface layer having an opticalsurface that is of fire-polished quality; and a bottom layer disposed ona second side of the core layer; wherein the core layer has a highersection modulus than the surface layer and the bottom layer to increasean overall stiffness of the laminated sheet.
 40. The laminated sheet ofclaim 39, wherein the optical surface is a fusion-formed surface. 41.The laminated sheet of claim 39, wherein a thickness of the surfacelayer and the bottom layer is about 1 mm or less.
 42. The laminatedsheet of claim 39, wherein the surface and bottom layers are each madeof a viscoelastic material.
 43. The laminated sheet of claim 42, whereinthe viscoelastic material is selected from the group consisting ofglasses and polymers.
 44. The laminated sheet of claim 39, wherein thecore layer is made of a viscoelastic material having a low thermalshrinkage.
 45. The laminated sheet of claim 39, wherein the core layeris made of an annealed glass.
 46. The laminated sheet of claim 39,wherein the core layer is made of an expandable material.
 47. Thelaminated sheet of claim 39, wherein the core layer comprises aglass-ceramic.
 48. The laminated sheet of claim 39, wherein the bottomlayer has a fusion-formed optical surface of fire-polished quality. 49.A method of making a laminated sheet, comprising: supplying a firstviscous, flowable material into an overflow channel; overflowing thefirst viscous, flowable material in a controlled manner to form a firstsheet-like flow having at least one untouched surface of fire-polishedquality; depositing at least a second material on a surface of the firstsheet-like flow to form a laminated flow; and drawing the laminated flowinto a laminated sheet.
 50. The method of claim 49, wherein depositingthe second material comprises supplying the second material in viscous,flowable form into an overflow channel and allowing the second materialto overflow in a controlled manner to form a second sheet-like flowwhich is merged with the first sheet-like flow. to form the laminatedflow.
 51. The method of claim 49, wherein depositing the second materialcomprises spraying the second material on the first sheet-like flow. 52.The method of claim 51, wherein the second material comprises a curablematerial.
 53. The method of claim 52, further comprising curing thecurable material while drawing the laminated flow into a laminatedsheet.
 54. The method of claim 49, wherein the first viscous, flowablematerial comprises one selected from the group consisting of polymersand glasses.
 55. The method of claim 54, wherein the second viscous,flowable material comprises one selected from the group consisting ofpolymers and glasses.
 56. A method of making a laminated sheet,comprising: supplying a viscous, flowable material into an overflowchannel; positioning a consumable material in a flow path of theviscous, flowable material so as to introduce a compositional variationto a specific portion of the viscous, flowable material; overflowing theviscous, flowable material in a controlled manner to form a sheet-likeflow having at least one untouched surface of fire-polished quality; anddrawing the sheet-like flow into a laminated sheet.
 57. The method ofclaim 56, wherein the consumable material comprises a nucleating agentfor a glass-ceramic.
 58. A method of making a laminated sheet,comprising: forming a first sheet-like flow by a continuous sol-gelprocess; supplying a viscous, flowable material into an overflowchannel; overflowing the viscous, flowable material in a controlledmanner to form a second sheet-like flow; fusing the second sheet-likeflow with a surface of the first sheet-like flow to form a laminatedflow; and drawing the laminated flow into a laminated sheet.
 59. Amethod of making a laminated sheet, comprising: supplying a first and asecond viscous, flowable material into independent compartments in anoverflow channel; simultaneously overflowing the first and secondviscous, flowable materials to form separate flow streams; uniting theseparate flow streams into a single sheet-like laminated flow; anddrawing the sheet-like laminated flow into a laminated sheet.
 60. Amethod of making a laminated sheet, comprising: drawing a first viscous,flowable material through a slot to form a first sheet-like flow;supplying a second viscous, flowable material into an overflow channeland overflowing the second viscous, flowable material from the overflowchannel in a controlled manner to form a second sheet-like flow havingan untouched surface of fire-polished quality; merging the secondsheet-like flow with a surface of the first sheet-like flow to form alaminated flow; and drawing the laminated flow into a laminated sheet.61. A method of making a laminated sheet, comprising: supplying aviscous, flowable material into an open channel; simultaneouslyoverflowing the viscous, flowable material from a first side of the openchannel and drawing the viscous, flowable material from a second side ofthe open channel; wherein the viscous, flowable material drawn from thesecond side of the open channel forms a sheet-like flow; wherein theviscous, flowable material overflowed from the first side of the openchannel forms two separate flow streams having untouched surfaces offire-polished quality; merging each of the flow streams with a surfaceof the sheet-like flow to form a laminated flow; and drawing thelaminated flow into a laminated sheet.
 62. A method of making alaminated sheet, comprising: obtaining a sheet of material; supplying afirst viscous, flowable material to a first overflow channel;overflowing the first viscous, flowable material in a controlled mannerto form a sheet-like flow with an untouched surface of fire-polishedquality; and while the sheet-like flow is in viscous, flowable form,merging the sheet-like flow with a surface of the sheet of material toform a laminated sheet.
 63. The method of claim 62, further comprisingdrawing the laminated sheet to a desired thickness.
 64. The method ofclaim 63, further comprising supplying a second viscous, flowablematerial to a second overflow channel, overflowing the second viscous,flowable material in a controlled manner to form a sheet-like flow withan untouched surface of fire-polished quality, and, while the sheet-likeflow is in viscous, flowable form, merging the sheet-like flow with asecond surface of the sheet of material.
 65. The method of claim 63,wherein the sheet of material comprises an annealed glass.
 66. Themethod of claim 63, wherein the sheet of material comprises anexpandable material.
 67. The method of claim 63, wherein the sheet ofmaterial comprises a glass-ceramic.
 68. A method of making a laminatedsheet, comprising: supplying a first viscous, flowable material to afirst overflow channel and a second viscous, flowable material to asecond overflow channel; overflowing the first viscous, flowablematerial from the first overflow channel in a controlled manner to forma first sheet-like flow with at least one untouched surface offire-polished quality; overflowing the second viscous, flowable materialfrom the second overflow channel in a controlled manner to form a secondsheet-like flow with at least one untouched surface of fire-polishedquality; drawing the first sheet-like flow into a first sheet and thesecond sheet-like flow into a second sheet; feeding the first sheet intoa float bath and continuously moving the first sheet across the floatbath; and feeding the second sheet into the float bath and merging thesecond sheet with the first sheet so as to form a continuously movinglaminated sheet across the float bath.
 69. An apparatus for forming alaminated sheet, comprising: an overflow means for forming opposedsheet-like flows, the overflow means having at least two independentcompartments for receiving a viscous, flowable material; a forming meanspositioned below the overflow means for converging and uniting theopposed sheet-like flows into a single drawn sheet flow; means forpivotally adjusting the overflow means such that a surface of theoverflow channel has a desired tilt angle with respect to thehorizontal; and means for delivering a viscous, flowable material intothe compartments.
 70. An apparatus for forming a laminated sheet,comprising: an overflow channel for forming opposed sheet-like flowsfrom a viscous, flowable material; a slotted channel positioned belowthe overflow channel for forming a drawn sheet-like flow; a forming bodypositioned below the overflow channel for converging and uniting theopposed sheet-like flows with outer surfaces of the drawn sheet-likeflow to form a laminated flow; means for pivotally adjusting theoverflow channel such that a surface of the overflow channel has adesired tilt angle with respect to the horizontal; and means fordelivering a viscous, flowable material into the overflow channel. 71.The apparatus of claim 70, wherein the slotted channel is fluidlyconnected to the overflow channel so as to receive viscous, flowablematerial from the overflow channel.
 72. The apparatus of claim 70,further comprising means for delivering a viscous, flowable material tothe slotted channel.
 73. The apparatus of claim 70, wherein the slottedchannel is formed in the forming body.